This specification relates to determining a resource's reachability score.
The Internet provides access to a wide variety of resources, including web pages, images, text documents, and multimedia content. Search engines identify resources in response to a query including one or more search terms or phrases by returning search results that link to the identified resources. The search results are typically ordered for viewing according to scores assigned to the search results by a scoring function. The scoring function ranks the search results according to various signals including, for example, the relevance of the resources to the query and measures of quality of the resources.